Re: replacing drives

[Robin Hill <robin@robinhill.me.uk>]

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On Fri May 03, 2013 at 06:28:02PM +0200, Roberto Nunnari wrote:

> Robin Hill wrote:
> > The safest option would be:
> >  - add in the new disks
> >  - partition to at least the same size as your existing partitions (they
> >    can be larger)
> >  - add the new partitions into the arrays (they'll go in as spares)
> >  - grow the arrays to 4 members (this avoids any loss of redundancy)
> >  - wait for the resync to complete
> >  - install grub/lilo/syslinux to the new disks
> >  - fail and remove the old disk partitions from the arrays
> >  - shrink the arrays back down to 2 members
> >  - remove the old disks
> > 
> > Then, if you're keeping the same number of partitions but increasing the
> > size:
> 
> Ok.. got here.
> 
> >  - grow the arrays to fill the partitions
> >  - grow the filesystems to fill the arrays
> 
> Now the scary part.. so.. here I believe I should give the following 
> commands:
> 
> mdadm --grow /dev/md0 --size=max
> mdadm --grow /dev/md1 --size=max
> mdadm --grow /dev/md2 --size=max
> 
Yep, that's right. Make sure they've actually grown to the correct size
before you progress though - I have had one occasion where using
--size=max actually ended up shrinking the array and I had to manually
work out the size to use in order to recover. That was using an older
version of mdadm though, and I've not seen it happen since.

> and after that
> 
> fsck /dev/md0
> fsck /dev/md1
> fsck /dev/md2
> 
You'll need 'fsck -f' here to force it to run.

> and
> 
> resize2fs /dev/md0
> resize2fs /dev/md1
> resize2fs /dev/md2
> 
> Correct?
> 
That should be it, yes.

> 
> .. I still have a couple of questions:
> 
> 1) how do I know if there's a bitmap?
> 
Check /proc/mdstat - it'll report a bitmap - e.g.
md6 : active raid6 sdg[0] sdf[6] sde[5] sdi[2] sdh[1]
      11721052272 blocks super 1.2 level 6, 16k chunk, algorithm 2 [5/5] [UUUUU]
      bitmap: 0/30 pages [0KB], 65536KB chunk

> 2) at present /dev/md2 usage is 100%.. could that cause any problem?
> 
It'll slow things down a bit but otherwise shouldn't be an issue.

> 3) the new drives are 2TG drives.. As around one year ago had trouble on 
> linux (it was a server dated 2006 with CentOS 5) that would not handle 
> drives larger than 2TB.. I wander what happens if one day one drive 
> fails and the drive I'll buy to replace will be sold as 2TB but in 
> reality slightly larger than 2TB.. what will happen? Will linux fail 
> again to use a drive larger than 2TB?
>
All 2TB drives are exactly the same size. Since somewhere around the
320G/500G mark, all drive manufacturers have agreed to standardise the
drive sizes, so getting mismatches like this is a thing of the past.

> At present I'm on ubuntu 10.04, all software from standard distribution.
> 
> Pitfalls I should know?
> 
You'll need to use GPT partitions instead of standard MBR partitions for
drives over 2TB, but there shouldn't be any issue with handling them.

Cheers,
    Robin
-- 
     ___        
    ( ' }     |       Robin Hill        <robin@robinhill.me.uk> |
   / / )      | Little Jim says ....                            |
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